1. Field of the Invention
This present invention relates to a power-scalable hybrid algorithm for smoothing artificial discontinuities between image blocks (blocking artifacts) and reducing ringing noise, without introducing undesired blur. More particularly, the algorithm of the present invention performs compressed-domain-based de-blocking in areas where blocky artifacts are most noticeable, which may be followed by spatial domain processing in which further de-blocking and de-ringing are performed, subject to need and available resources such as battery or computation power. The invention, which can be embodied in an apparatus, methods or programs of instructions, is particularly, though not exclusively, designed for low-bit-rate video coding applications.
2. Description of the Related Art Recently, the emerging low bit-rate video coding schemes, such as ITU-T H.263 and MPEG-4, have required more efficient and effective algorithms to reduce annoying blocking and ringing artifacts. By the nature of block-based coding, monotone areas of the original image, where the pixel intensity is changing gradually, suffer most noticeably from the abrupt changes across the block boundary, leading to blocking artifacts. In terms of the discrete cosine transform (DCT), when the DCT coefficient quantization step size is above the threshold for visibility, discontinuities in grayscale values are caused by removal of AC coefficients due to quantization and become clearly visible at the boundaries between blocks. Also, visually annoying noise may exist near real edges. Such noise is called ringing noise and is inevitable in the block-based discrete cosine transform (DCT) scheme. Since these artifacts cause substantial practical problems, reduction of those artifacts is essential in applications using low bit-rate coded video sources, such as mobile terminals including video phones and PDAs. Many de-blocking schemes have been proposed in still image coding, such as JPEG, under the assumption that blocking artifacts are always located at block boundaries. A well-known method for reducing blocking artifacts is based on the theory of alternative projection onto convex sets (POCS). However, this method can only be applied to still images because of an iteration structure and long convergence time.
Recently, post-filtering methods for low bit-rate coded video have been proposed. In video coding, unlike in still image coding, blocking artifacts may occur inside a block since the blocking artifacts of the previous frame can be propagated to arbitrary positions in the current frame. Selective filtering methods using local characteristics have been proposed. The basic idea is to classify pixels or blocks into two categories in order to apply filters adaptively. One reported method involves a de-blocking filter with two separate modes, which are selected on the pixel behavior around the block boundary. However, reducing ringing noise is not considered. In another proposal, a frequency-domain analysis is conducted for extraction of the block semaphores from the intracoded picture. The semaphores extracted are propagated using inverse motion compensation for inter-frame de-blocking and de-ringing.